New approach of Indian monsoon estimation to account for rainfall variations

The India Meteorological Department (IMD)’s declaration of India’s overall monsoon conditions, as drought, surplus, and normal, at the end of the season often ignores the rainfall variations over different times and various places throughout the country, says a new study, questioning IMD’s methodology of estimating Indian summer monsoon conditions. The study suggested a new framework for defining the nature of summer monsoons.

This new framework developed by scientists from the Indian Institute of Technology (IIT), Gandhinagar and Helmholtz Centre for Environmental Research in Leipzig, Germany, can account for rainfall variations over time and geography and account for extreme weather events within the season and their implications for agriculture and water management. In devising the new framework, they found that while 84 years were declared normal in the last 121 years (1901–1921), 13 years (out of 84) were not normal, based on the new framework, due to dry and wet extremes occurring at different times and in different regions.

The study authors pointed out that the current method used by the IMD to estimate the conditions of the summer monsoon does not account for variation across geography and time, and the approach ignores wide variations of rainfall over different times in the monsoon period as also variations in the different regions of the country.

So, dry and wet extremes within the same monsoon season can lead to estimations of a normal monsoon. Also, while different parts of the country face drought and wet extremes, the summer monsoon can be declared normal.

As a test case, the scientists examined India’s 2021 summer monsoon season rainfall which had been declared normal and found that weekly rainfall in that year reflected both dry and wet extremes during the season. Wet conditions prevailed largely in the early summer monsoon season, followed by the two long rainfall breaks during July and August. Extreme precipitation in several parts of the country created wet conditions at the end of the summer monsoon season. The same monsoon season witnessed extreme dry and wet conditions.

“Several parts of the country, especially central India, experienced extreme to exceptional category drought during the fifth and tenth week of the 2021 summer monsoon season. In contrast, a large part of the country experienced extreme wet conditions during the 17th and 18th weeks of the monsoon season,” the authors write in the paper.

The India Meteorological Department (IMD) monitors the progress and state of the summer monsoon every year. At the end of the monsoon season, IMD declares the overall condition of the summer monsoon as drought (precipitation deficit), surplus or normal. Since summer monsoon rainfall generally experiences a ten percent deviation from the long-term average, a threshold of ten percent is used to declare drought or surplus monsoon.

A ten percent departure of all-India average precipitation from its long-term mean is declared a normal summer monsoon. If the rainfall deficit at the end of the season is more than ten percent, the year is declared a drought. On the other hand, if the summer monsoon rainfall exceeds 10%, the monsoon is surplus. Therefore, this declaration of different states of the summer monsoon is based on all-India averaged rainfall.

Flaws in the estimation of monsoon rainfall can affect research, prediction and planning in a country for which the season forms the lifeline for millions of people’s water availability and agricultural activities. More so, as several parts of the country, including the Indo-Gangetic Plain, Western Ghats, and northeast India, have experienced a significant decline in the summer monsoon rainfall over the last few decades affecting water resources and food production. On the other hand, studies have shown that parts of western and central India have experienced an increase in summer monsoon rainfall primarily due to extreme rainfall, which has resulted in flooding. Additionally, the variability of rainfall extremes during the monsoon season is seen more often, causing increasing dry and wet spell frequency.

The summer monsoon season (June-September), when India receives 80% of the total annual rainfall, overlaps with one of the major crop-growing seasons (kharif), and monsoon rain is a prominent water source for supporting agricultural activities. These variations have underscored the need for a revised system of estimation of the monsoon, which is central to the life and livelihood of many.

The three-step criteria

The authors recommend three criteria should be met for a monsoon to be declared normal – all-India averaged summer monsoon rainfall departure, standardised departure of Combined Severity Coverage Index (CSCI) and cluster-wise approach.

For a monsoon in a certain year to be called a normal, all-India averaged summer monsoon rainfall, departure should be within ten percent, standardised departure of Combined Severity Coverage Index – a measure of the spread of severe drought and wet events – should be less than one, and not more than three clusters should be under drought or extreme wet conditions.

Elaborating on the research, study author Vimal Misra said, “We obtained daily gridded precipitation at 0.25° spatial resolution, which was developed using more than 6900 observational stations across India.”

They estimated precipitation anomalies for each week during the summer monsoon season to examine the temporal variability of the monsoon. Weekly precipitation anomalies were estimated against the long-term (1901–2021) mean of rainfall for the corresponding weeks.

Correction in monsoon estimation over the years

“Monsoon exhibits huge variability on regional and temporal scales. For example, during the 2022 monsoon onset time, northwest and northern states witnessed dry conditions and a delay in the monsoon, affecting agriculture in the region. At the same time, the northeastern states experienced extreme rainfall events leading to floods that claimed lives and livelihoods,” said Roxy Mathew Koll, a climate scientist at the Indian Institute of Tropical Meteorology, who was not associated with the paper.

“Despite these regional differences, the monsoon is declared normal by the IMD if the total numbers across all the states in the country add up to near-average. Declaring the monsoon normal in such a way can be misleading since different regions in the country can experience wet spells or dry conditions at the same time or during different months in the monsoon season. This can lead to regional droughts or floods that need to be acknowledged and studied and not forgotten. If these regional droughts or floods appear repeatedly over the years due to climate change (as is happening), we would need policies at a regional level,” he added.

The proposed framework can be used to examine the spatial and temporal variability of extremes during the monsoon season for any year, regardless of whether a year is normal. Regular estimation of the all-India averaged summer monsoon rainfall using this new framework could help in improving the accuracy of research and future forecasts, especially in a scenario where climate change has made extreme events more frequent.

“I appreciate the new method of estimating the monsoons, but we at IMD also take into consideration the temporal variations of rainfall in the wet and dry spells through daily and weekly rainfall records and spatial variations of rainfall through state and district level rainfall records. These are integrated into the estimations of monsoons and for declaring their status in different time intervals during the monsoon season and at the end of the season. Thus, monsoon status is also estimated and declared region-wise,” Mrutyunjay Mohapatra, Director General of India Meteorological Department (IMD), told Mongabay-India.

Banner image: Southwest monsoon clouds in the Western Ghats. Photo by Nikhilrabade/Wikimedia Commons.